1. Field of the Invention
The present invention relates to devices for displaying images, and more particularly, to a device and a method of driving a light source for image display devices.
2. Description of the Related Art
Display devices, such as computer monitors, television sets, etc., generally include self-emitting display devices employing light emitting diodes (LEDs), electroluminescences (ELs), vacuum fluorescent display (VFD), field emission display (FED) and plasma panel display (PDP), and non-emitting display devices employing liquid crystal display (LCD) which necessitates a light source.
LCD devices are generally equipped with two panels each having a field-generating electrode and a liquid crystal (LC) layer with dielectric anisotropy. The LC layer is interposed between the two panels. The field-generating electrodes are each supplied with an electric voltage to generate electric fields across the liquid crystal layer. The light transmittance of the LC layer varies in association with the strength of the electric fields, which is controlled by the applied voltage. Accordingly, desired images are displayed by adjusting the applied voltage.
The light for an LCD device is provided by a light source equipped within the LCD device or may be the natural light. In case of employing a light source to provide the light, the brightness on a screen of the LCD device is usually adjusted by regulating the ratio of on- and off-time of the light source or regulating the current flowing the light source.
As a light source for the LCD devices, fluorescent lamps are usually used. The fluorescent lamps generally require a high AC voltage of which magnitude is typically in the range of several kilovolts and frequency in the range of dozens of kilohertz. The current flowing such fluorescent lamps has a magnitude of several milli-amperes. Since the lamps are disposed at the rear side of an LCD panel and close to the panel at a distance of several millimeters, electric fields and magnetic fields from the lamps make noise to signals in wires and thin film transistors (TFTs) of the LCD panel. In particular, since the frequency of a driving signal for the lamps and the frequency of a horizontal synchronization signal for the LCD panel are similar to each other but a slight difference, a beating occurs to cause interference which makes horizontal stripes, called waterfall, on the LCD screen.
In order to coinciding the frequency of the lamp driving signal and the frequency of the horizontal synchronization signal for removing such problems, a triangular pulse width modulation (PWM) reference signal having a frequency lower than the frequency of the horizontal synchronization signal is generated and the reference signal is dropped to a bottom level at the time of synchronization by using short pulses to be initiated.
However, in the display devices employing the conventional light source driving devices and method, the triangular reference wave is generated to have rising portions and falling portions, which are asymmetric to each other. As a result, are caused several problems such as reduced lifetime and unstable ignition of the lamps.